1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing roller, a fixing belt wound around the fixing roller, and a pressing roller pressed against the fixing roller via the fixing belt to form a nip portion between the fixing belt and the pressing roller through which a sheet bearing a toner image passes. The fixing belt and the pressing roller apply heat and pressure to the sheet bearing the toner image as the sheet passes through the nip portion to fix the toner image on the sheet. Thereafter, the sheet bearing the fixed toner image is separated from the fixing belt and the pressing roller by a separator contacting the fixing belt, and conveyed to the outside of the image forming apparatus.
However, the separator that contacts the fixing belt to separate the sheet bearing the fixed toner image from the fixing belt is capable of scratching and damaging the fixing belt. When the damaged fixing belt contacts the fixed toner image on the sheet, especially a glossy color toner image, scratches on the fixing belt may spoil the image. To address this problem, a non-contact separator may be provided to separate the sheet bearing the fixed toner image from the fixing belt without contacting the fixing belt. However, in this case, the fixing belt is required to have a greater-than-usual degree of curvature to facilitate separation of the sheet from the fixing belt.
On the other hand, increasing demand for high-speed toner image formation requires that the nip portion formed between the fixing belt and the pressing roller have a greater length in a sheet conveyance direction, so that sufficient heat and pressure can be applied to the sheet to securely fix the toner image even when the fixing belt and the pressing roller rotate at faster speeds.
To address such requirements, the fixing belt may be wound around a plurality of rollers to provide the greater curvature required to facilitate separation of the sheet from the fixing belt and the greater length of the nip portion. For example, a separation roller may be provided downstream from the nip portion formed between the fixing roller and the pressing roller in the sheet conveyance direction, and pressed against the pressing roller via the fixing belt to form a second nip portion between the separation roller and the pressing roller. Thus, the first nip portion formed between the fixing roller and the pressing roller and the second nip portion formed between the separation roller and the pressing roller provide the greater nip length needed to apply sufficient heat and pressure to the sheet. Also, the fixing belt wound around the fixing roller and the separation roller provides the greater curvature to facilitate separation of the sheet from the fixing belt.
However, there are drawbacks to the above-described configuration. For example, at an intermediate nip portion provided between the first nip portion and the second nip portion in the sheet conveyance direction, the fixing belt may not be pressed against the pressing roller properly, and may separate from the pressing roller. Further, a circumferential velocity of the fixing belt may differ from a circumferential velocity of the pressing roller due to reduced friction between the fixing belt and the pressing roller at the intermediate nip portion. Consequently, the fixing belt may become slack or slip, resulting in formation of a faulty toner image.